Radio frequency power amplifiers have a nonlinear power transfer function. In other words, both the amplitude and phase components of the amplifier's gain depend on the power of the amplifier's input signal. This amplifier non-linearity is undesirable because it distorts the output waveform, broadens the output waveform's spectrum, and generates interference within, adjacent channels.
Generally, power amplifier linearization is sought using one of three techniques: vector feedback, adaptive predistortion, and adaptive feedforward compensation. In each of the three techniques, the amplifier's complex gain error must be estimated, preferably in real-time. It is toward this estimation step that the present invention is directed.
Conventionally, the gain error is estimated by comparing the amplifier's output signal with a reference signal, usually the amplifier's input signal. This comparison measures quantities such as the correlation or the error power of the two signals.
Unfortunately, conventional correlation circuits introduce DC offsets and common-mode feedthrough errors. Also problematic, conventional error power measurement circuits use a search process that disturbs the gain along the amplification path and yields a signal convergence that is too slow for vector feedback systems.
It would be desirable to have a gain error estimator that does not suffer from these problems.